Prognostic Meteorological Discussion
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FXUS02 KWBC 310521
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
120 AM EDT FRI JUL 31 2015

VALID 12Z MON AUG 03 2015 - 12Z FRI AUG 07 2015

...OVERVIEW...
FLOW PATTERN ACROSS CANADA BEGINS TO TAKE ON AN `AMPLIFIED`
APPEARANCE DURING THIS MEDIUM RANGE PERIOD---WITH A KEY COMPONENT
OF THE PATTERN---AN ANCHORING UPPER-LEVEL LOW AND TROUGH SPREADING
ACROSS EASTERN NORTH AMERICA. THIS TROUGH WILL BE RESPONSIBLE FOR
BELOW NORMAL TEMPERATURES ACROSS THE GREAT LAKES AND NORTHEAST ---
DURING THE LATTER HALF OF THE MEDIUM RANGE PERIOD.

...MODEL PREFERENCES/UNCERTAINTY ASSESSMENT...
THOUGHT THE 30/12Z DETERMINISTIC ECMWF...CANADIAN AND GFS WERE
`USE-ABLE` PIECES OF GUIDANCE THROUGH 4/12Z---DEPICTING TWO
NOTABLE 500MB CIRCULATIONS. ONE NEAR JAMES BAY CANADA. THE SECOND
CIRCULATION---LODGED INVOF HAIDA GWAII/SOUTHERN PRINCE OF WALES
ISLAND AK.

THE WEST COAST CIRCULATION WILL BE THE TROUBLESOME ONE BEYOND DAY
4---GIVEN THE NORTHERN PACIFIC `PROGRESSIVENESS`---A STAPLE OF THE
UNUSUAL NORTH PACIFIC PATTERN---FOR THE 7-14 DAYS--- HAS COME TO
AN END. DISAPPEARED ESSENTIALLY...ACROSS CENTRAL CANADA---WITH THE
ARCTIC LOW HAVING AMPLE TIME TO CONSOLIDATE ACROSS THE YUKON AND
NORTHWEST TERRITORIES BY PERIOD`S END.

WITH NO REAL UPSTREAM `KICKER` TO SPEAK OF---THIS 500MB
CIRCULATION WILL HAVE ALOT OF ROOM TO `MEANDER` ALONG THE WEST
COAST OF NORTH AMERICA. THE BEST ESTIMATION (AT THIS JUNCTURE) TO
THE TRUE NATURE OF ITS `MEANDERING` TRAJECTORY AND SENSIBLE
WEATHER IMPACTS WOULD BE THE DEPICTION OFFERED BY THE 30/12Z
ECMWF/GFS DETERMINISTIC SOLUTIONS HEADING THROUGH THE MID-POINT OF
DAY 6 (6/12Z). IF THESE SOLUTIONS CAN BE CONSIDERED REASONABLE
DEPICTIONS OF THE FLOW---A SERIES OF MID-LEVEL SHORTWAVES WILL
MIGRATE (IN A SOUTHWEST FLOW) ACROSS CALIFORNIA...THE NORTHERN
GREAT BASIN AND NORTHERN ROCKIES---AHEAD OF THE 500MB TROUGH AXIS.
EITHER THE 31/12Z OR 31/18Z GEFS MEAN...IN A BLEND WITH THE 31/12Z
ECENS MEANS WOULD BE SUITABLE---IF NOT SOLID `PLACE SETTERS`---
FOR THE UPCOMING FLOW PATTERN AMPLIFICATION FROM WEST TO EAST
ACROSS THE CONTINENT.

EVENTUALLY THOUGH---IT WILL BECOME--- ONE OR THE OTHER ALONG THE
WEST COAST---BUT IT CANNOT BE BOTH. MEANING---BLENDING THE
GEFS/ECENS MEANS WILL BECOME AN INEFFECTIVE FORECAST METHOD ACROSS
THE PACIFIC NORTHWEST AND DOWNSTREAM ALONG 100W LONGITUDE. FOR THE
TWO CHOICES ARE---A CONTINUED PROGRESSION OF NORTHEAST PACIFIC
FLOW INTO WEST CENTRAL CANADA OR THE END OF IT. WAS ESSENTIALLY
LEFT WITH A MANUAL EDIT FOR MY DAY 6 SURFACE GRAPHIC. THIS LEFT
SOME ROOM FOR THE GEFS/GFS APPROACH---LETTING A SHORTWAVE MIGRATE
NORTHEASTWARD ACROSS THE GULF OF ALASKA...AND `KICK` A LARGE
PORTION OF THE DOWNSTREAM TROUGH INLAND ACROSS THE INTERMOUNTAIN
WEST. THIS WILL ---FOR ALL INTENTS AND PURPOSES --- BEGIN TO
DENIGRATE THE CONTINUITY OF THE `PACIFIC FRONT` BY DAY 6 AS IT
REACHES THE NORTHERN DIVIDE---SINCE EVERYTHING DOWNSTREAM IS
POINTING TO A `BLOCKY` MID-LEVEL AND AMPLIFYING SHORTWAVE RIDGE
OVER SOUTHERN SASKATCHEWAN/MANITOBA ---WITH BROAD NORTHWEST FLOW
PREVAILING WITH ORIGINS---DIRECTLY OUT OF THE NORTHWEST
TERRITORIES AND OVERSPREADING THE ENTIRE NORTHEAST QUARTER OF THE
LOWER 48. THE TRANSITION REGION DOWNSTREAM ACROSS THE LOWER 48
(DAYS 6-7) WILL BE THE MISSOURI AND PLATTE VALLEYS---WHERE THE
MID-LEVEL NORTHWESTERLY FLOW PERIODICALLY GENERATES A `BACKDOOR`
COLD FRONT WHICH ATTEMPTS TO MIGRATE FROM THE MIDWEST INTO THE
CENTRAL HIGH PLAINS. IF THERE IS ROOM FOR ERROR---IT WILL BE THE
TEMPERATURE DISTRIBUTION ON DAY 6 ALONG THE I-80 AND I-70
CORRIDORS FROM CENTRAL COLORADO TO THE MISSISSIPPI RIVER.

AT LEAST IN THE EASTERN PORTION OF THE CONTINENT...THE FORECAST
SEEMED A BUT MORE STABLE---WITH THE TROUGH ANCHORING A NORTHWEST
FLOW ALOFT ACROSS THE UPPER GREAT LAKES. HAVE DEPICTED THIS
SCENARIO WITH A SERIES OF SECONDARY COLD FRONTS AND TROUGHS---THAT
REINFORCE THE COOLER CANADIAN AIRMASS ACROSS THE GREAT LAKES AND
NORTHEAST THROUGH THE LATTER HALF OF THE FORECAST PERIOD.

THE CHALLENGING PART OF THE FORECAST EAST OF THE DIVIDE APPEARS TO
BE PRECIPITATION. WHAT ENERGY CAN SLIP DOWNSTREAM FROM THE
`PACIFIC FRONT` HAS LIMITED ACCESS---AND WILL ENCOUNTER A DRY AND
STABLE EASTERLY LOW-LEVEL FLOW ALONG THE LOWER HALF OF THE
MISSOURI AND WESTERN HALF OF THE OHIO VALLEYS. ANOTHER PROBLEM
AREA IS THE GULF COAST. THE `OLD` BAROCLINIC BOUNDARY AND
SUBTROPICAL EASTERLY TRADE WINDS APPEAR TO SHIFT THE INSTABILITY
INTO THE NORTH CENTRAL GULF OF MEXICO---AND GRADUALLY SEND THE
INSTABILITY NORTHWARD WITH THE WESTWARD `REBUILDING` OF THE
SUBTROPICAL SURFACE RIDGE (REGION OF GREATER INSTABILITY DRAWN ON
MY DAY 4 SURFACE GRAPHIC AS A TROUGH).

...SENSIBLE WEATHER HIGHLIGHTS...
A `COOL-DOWN` FOR THE NORTHEAST QUARTER OF THE NATION---OR AT THE
VERY LEAST...A MARKED DECREASE IN HUMIDITY! FOR THE INTERMOUNTAIN
WEST---HOT WITH SCATTERED HIGH-BASED...TERRAIN-DRIVEN CONVECTION
AND FOR THE SOUTHERN PLAINS...HOT AND HUMID WITH A GRADUAL
INCREASE IN WESTERN GULF INFLUENCES.
VOJTESAK

$$




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